(i) Field of the Invention
The present invention relates to a profiling control apparatus for controlling the profiling operation of a profiling machine without jumping of a stylus of a tracer head at a place where a surface shape of a model to be profiled is suddenly changed, and controlling a profiling speed of the profiling machine and a control method thereof.
(ii) Prior Art Statement
A profiling apparatus in the prior art includes a stylus 2a coupled with a tracer head 2 and pressed on a surface of a model 1 to be profiled as shown in FIG. 1, and the stylus 2a is adapted to profile or move along the surface of the model 1 with the movement of the tracer head 2. However, at a place where the surface shape of the model 1 is suddenly changed, for example at a step, when the tracer head 2 moves at a high speed, the stylus 2a can not follow the surface shape of the model 1 and accordingly jumps or floats from the surface of the model 1.
Such a jumping phenomenon of the stylus results in that the stylus 1 does not exactly profile or move along the surface of the model 1. Accordingly, it is required to perform the profiling operation along the surface of the model again with a reduced moving speed of the tracer head 2. However, there is no method of judging whether the jumping phenomenon has occurred or not heretofore.
Further, when the profiling speed is high, the jumping phenomenon of the stylus 2a occurs and hence it is difficult to exactly perform the profiling operation along the shape of the model.
FIG. 2 is a block diagram showing an example of a prior art profiling control apparatus. In FIG. 2, numeral 2a denotes a stylus, numeral 2 denotes a tracer head which detects displacement signals .epsilon.x, .epsilon.y and .epsilon.z of the stylus 2, numeral 3 denotes a vector arithmetic unit which calculates a profiling speed vector V on the basis of the displacement signals .epsilon.x, .epsilon.y and .epsilon.z of the stylus 2a and an established profiling velocity V.sub.T and produces the profiling speed vector V, and numeral 4 denotes an amplifier which amplifies component Vx, Vy and Vz of the vector V, respectively, and drives a motor 5 for each shaft.
Operation of the apparatus in FIG. 2 is now described with reference to FIGS. 3 and 4. The displacement signals .epsilon.x, .epsilon.y and .epsilon.z of the stylus 2a which follows the surface of the model 1 shown in FIG. 3, are applied to the vector arithmetic unit 3 through the tracer head 2. The vector arithmetic unit 3 calculates compound displacements .epsilon. and .vertline..epsilon..vertline.=.sqroot..epsilon.x.sup.2 +.epsilon.y.sup.2 +.epsilon.z.sup.2 of the displacement signals .epsilon.x, .epsilon.y and .epsilon.z to obtain a difference signal .epsilon.-.epsilon.o between a given reference displacement .epsilon.o and the compound displacement and produces a preset profiling velocity V.sub.T in a direction parallel with the surface of the model 1 perpendicular to the direction of the vector .epsilon. perpendicular to the surface of the model 1, to compound the signal .epsilon.-.epsilon.o and the velocity V.sub.T vectorially to obtain an actual profiling vector V. The vector arithmetic unit 3 produces components Vx, Vy and Vz of the vector, V which are amplified by the amplifiers 4 and control each shaft of the drive motors 5.
Such a profiling control apparatus can not control the profiling operation to follow the surface of the model at a place where the surface shape of the model 1 is suddenly changed. Accordingly, the fact that the signal .epsilon.-.epsilon.o is increased at the suddenly changing portion, is utilized to produce a profiling setting velocity V.sub.T of the vector arithmetic unit 3 inversely proportional to the signal .epsilon.-.epsilon.o as shown in FIG. 4, so that the profiling velocity at the suddenly changing place of the surface shape of the model is reduced.
While the sudden variation of the displacement is detected to reduce the profiling velocity in this way in the above prior art apparatus, since the profiling velocity is reduced at the suddenly changing place of the surface shape of the model, it is too late due to influence of vibration or delayed response of a servo system in order to perform the exact profiling operation. Accordingly, it is impossible to establish largely a maximum value of the set velocity V.sub.T of the vector arithmetic unit (apex velocity of FIG. 4). Consequently, it is disadvantageous that the profiling velocity is suppressed to a low velocity as a whole. That is, the profiling time is made longer.
FIG. 5 is a block diagram showing a profiling control apparatus of a prior art. In FIG. 5, a surface of a model 1 is profiled or followed by a tracer head 2 in accordance with control of a digitizer 13. A profiled locus is converted to an NC data 14 by the digitizer 13 and the data is applied to an NC device 15. Then, a processing device 16 cuts a work 17 into the same shape as that of the model 1. In this profiling process, since the profiling speed is not controlling in accordance with the shape of the model 1 heretofore, a servo system of the NC device 15 can not follow the suddenly changing surface shape of the model 1, and accordingly overcut portions or uncut portions are produced. In order to prevent the overcut portions and the uncut portions heretofore, the processing speed is varied to be reduced to a low speed manually. However, when the processing speed is varied to be reduced to a low speed manually in this way, the processing efficiency is deteriorated.
Generally, a differential transformer is used to detect a displacement of the profiling operation in the profiling machine. However, while the profiling operation with accuracy of one hundredth in the full scale is merely attained in the case of the differential transformer, the above profiling machine attains the profiling operation with high accuracy of the order of several microns. A detection range of displacement in the profiling operation using the differential transformer is maximum +/-2 mm or so. However, there is a fear that an overshoot exceeding the detection range of the differential transformer occurs, for example, at a corner where the shape of the model is suddenly changed. When the overshoot occurs, the profiling machine can not be controlled. If a detector for the large scale displacement having a detection range capable of detecting the above overshoot is manufactured, it is very large in structure and expensive. Accordingly, when there occurs the overshoot exceeding the detection range, a manner of stopping the profiling operation of the profiling machine has been taken heretofore.
However, such a control method limits the profiling speed due to the detection range, and accordingly the high speed profiling operation which is desired in recent years can not be attained.